[2019-06] [System.Net.Http]: Add hack to the legacy HttpClient to pass down Timeout property. #12577. (#15014)
* [System.Net.Http]Add hack to the legacy HttpClient to pass down Timeout property. #12577.
* `HttpClientHandler`, `IMonoHttpClientHandler`: add new internal `MonoSetTimeout (TimeSpan)` function.
* `HttpClient.SendAsyncWorker()`: if `handler` is `HttpClientHandler`, call the new internal
`MonoSetTimeout()` function to pass down the timeout.
* `MonoWebRequestHandler.SendAsync()`: this is only used here to pass the timeout to the `HttpWebRequest`.
* Address feedback.
* Update HttpMessageInvoker.cs
* Make it build.
* Really make it build (forgot monotouch_watch).